This invention relates generally to a method of drying materials, and more particularly, is directed to a method for the drying of a material by controlling enthalpy and drying potential of a drying gas.
The ability of relatively dry air to extract moisture from surfaces with which it makes contact is employed in a wide range of industrial and other drying operations. These operations generally use the drying potential of the air to control moisture removal. The drying potential of air is the capability of air to supply energy both to evaporate water from a system and to carry the water away, where the air is the sole means of supplying such energy. The drying potential of air is determined solely by its temperature and moisture content. For example, saturated air at 30.degree. C. has no drying potential, while air at 75.degree. C. and 10% humidity has a drying potential. A consideration that must be taken into account in any such system however, is that more heat must be applied to produce drier air, which thereby affects the efficiency of the operation.
However, with various materials, different problems arise with these operations where only drying potential of the air is controlled. For example, in the drying of steeped grains and the like, it is known to use plural sensors to control the moisture content and temperature of the air in a batch drying operation. The problem with such operation is that, during the drying operation, different strata or layers within the grain are dried at different rates and times. As a result, there often occurs an insufficient or excessive drying and/or caramelizing (coloring) of the grain that results in, for example, low alcohol conversion and high colors, which are undesirable.
Specifically, if the bottom stratum or layer, for example, is heated too fast, moisture in the bottom layer evaporates and travels upwardly through the grain to the next or second layer. Since the second layer is at a lower temperature at this time, the large amount of moisture from the bottom layer condenses in the second layer. As a result, the air in the second layer becomes saturated and the second layer also contains excess condensed water. Since the air in the second layer is saturated, the excess condensed water in the second layer cannot evaporate. Effectively, this is because there is a greater enthalpy in the second layer than in the next upper or third layer. As the temperature rises, the hot, wet air causes stewing of the grain, which results in low alcohol yield. With respect to green malt, for example, the low alcohol yield results in sugars being destroyed, that is, converting sugars into caramels, which is detrimental to the brewing process.
On the other hand, if drying is too slow to avoid the above disadvantage, the drying operation is inefficient. It is therefore necessary to provide an efficient drying operation without damaging the grain.
With other materials to be dried, such as paper, an uneven pick up of starch at a late stage, for example, during sizing, may result, and/or the excessive drying may adversely affect the finishing of the paper.